1. Technical Field
The present invention relates to a wire bonding method and apparatus which gives a load and ultrasonic wave vibration between a wire and a connection pad to bond the wire to the connection pad. More particularly, the present invention relates to a wire bonding method and apparatus that is capable of managing an amount of transformation of the wire so that stable bonding strength can be obtained.
2. Description of the Related Art
Wire bonding methods for giving a load and ultrasonic wave vibration between a wire and a connection pad to bond the wire to the connection pad, have been widely used in fabricating electronic components such as semiconductor chips. FIG. 14 is a diagram showing the a wire bonded to a connection pad, with FIG. 14A showing the front view and FIG. 14B showing the top view. In this wire bonding method, the wire 1 is placed on the connection pad 2, and a bonding wedge 3 is brought into contact with this wire 1. Then, a load is exerted downward by the bonding wedge 3, whereby the load is given between the wire 1 and the connection pad 2. Next, the bonding wedge 3 is given ultrasonic wave vibration in the direction of arrow L shown in FIG. 14A, whereby ultrasonic wave vibration is given between the wire 1 and the connection pad 2.
With the load and the ultrasonic wave vibration, the wire 1 (indicated by the dashed line) is forced downward and spreads on both sides, and the contacting areas of the wire 1 and the connection pad 2 are bonded with each other. The upper surface 1a of the wire 1 is flattened and the width in the longitudinal direction of the wire 1 becomes substantially constant. The width nw of the wire 1 after transformation is called nugget width.
There are two methods of managing a load and ultrasonic wave vibration which are given between the wire 1 and the connection pad 2: a method of managing the time of giving a load and ultrasonic wave vibration; and a method of managing the transformation amount of a wire which is forced in the direction of exerting a load. In general, even if the same load and the same ultrasonic wave vibration are given, the transformation amount of a wire will vary. For this reason, the method of managing the transformation amount of a wire is able to transform a wire with a higher degree of accuracy. The wire bonding method, which manages the transformation amount of a wire, is disclosed, for example, in a microfilm for Japanese Utility Model Application No. 56-22139 (Japanese Utility Model Laid-Open No. 57-135736).
FIG. 15 is a side view of the wire and the connection pad of FIG. 14 taken in the direction of arrow A shown in FIG. 14. In the wire bonding method which manages the transformation amount of a wire, the movement quantity of the bonding wedge 3 is considered to be the transformation amount of the wire 1, and the movement quantity of the bonding wedge 3 is measured. Although the transformation amount of the wire 1 is ideal to measure the entire transformation amount c, practically an amount of transformation b is measured. An amount of transformation a indicates the quantity that the wire 1 is slightly transformed by the load exerted via the bonding wedge 3 when the bonding wedge 3 moving downward is brought into contact with the wire 1. This amount of transformation a is outside the management of the transformation amount of a wire.
A conventional wire bonding apparatus uses, for is example, the dead load of a horn (not shown), which supports the bonding wedge 3, to exert the load to the wire 1. If the bonding wedge 3 is lowered and brought into contact with the wire 1, the wire 1 is transformed in a moment. For this reason, it is difficult to set an amount of transformation to zero (initial value) by grasping the moment that the bonding wedge 3 is brought into contact with the wire 1. For this reason, this apparatus sets a time period enough for the bonding wedge 3 to abut and transform the wire 1 from the time when the bonding wedge 3 is lowered. At the time that the set time period has elapsed since the bonding wedge 3 was lowered, ultrasonic wave vibration is applied, and at the same time, the transformation amount of the wire is set to 0. In this way, the amount of transformation b is measured.
However, the conventional wire bonding method and apparatus, which manage the transformation amount of a wire, has the disadvantage that the entire transformation amount of the wire 1 will vary greatly, although the amount of transformation b is accurately managed to exert the load and ultrasonic wave vibration between the wire 1 and the connection pad 2.
It is considered that the variation in the entire transformation amount of the wire results from variations in the positional accuracy of setting works (connection pads and wires), the wire diameter, the thickness of the connection pad, the speed (shock load) at which the bonding wedge 3 abuts, and the like. However, after all, the variation in the transformation amount of the wire is due to the amount of transformation a, which is outside the management.
When transformation al is small, as shown in FIG. 16A, the entire transformation amount cl of the wire 1 becomes c1=a1+b. When transformation a2 is large, as shown in FIG. 16B, the entire transformation amount c2 of the wire 1 becomes c2=a2+b. The difference Da between the quantities of transformation a1 and a2, as it is, becomes the difference Dc between the quantities of transformation c1 and c2.
The present invention has been made in order to solve the aforementioned problems. Accordingly, the object of the present invention is to provide a wire bonding method and a wire bonding apparatus which reduces a variation in the entire transformation amount of a wire to enhance the reliability of the wire-bonding strength.
In accordance with one aspect of the present invention, there is provided a wire bonding method for bonding wires to connection pads, comprising: a loading step for transforming the wires by touching the wires to a bonding wedge to apply a predetermined load between the wires and connection pads; a time controlling step for providing a predetermined ultrasonic wave vibration with the predetermined load for a predetermined time through the bonding wedge after transforming the wires in the loading step; and a transformation amount controlling step for providing the predetermined ultrasonic wave vibration with the predetermined load between the wires and connection pads, while measuring an amount of wire transformation until the amount of wire transformation reaches a predetermined set value, the amount of wire transformation being set at an initial value, the transformation being directed to the loading after wearing the predetermined time in the time controlling step.
In accordance with another aspect of the present invention, there is provided a wire bonding apparatus for bonding wires to connection pads, comprising: load applying means for touching the wires to a bonding wedge to apply a predetermined load between the wires and connection pads; ultrasonic wave vibration providing means for providing a predetermined ultrasonic wave vibration between the wires and connection pads through the bonding wedge; transformation amount measuring means for measuring an amount of wire transformation; and control means for controlling operation of the load applying means, the ultrasonic wave vibration providing means and the transformation amount measuring means; wherein the control means performs control so that: first, the load applying means touches the wires to the bonding wedge, applies the predetermined load between the wires and connection pads through the bonding wedge, and transforms the wires; then, the load applying means applies the predetermined load for a predetermined time, and at the same time, the ultrasonic wave vibration providing means provides the predetermined ultrasonic wave vibration for the predetermined time; after the predetermined time, the transformation amount measuring means sets the amount of wire transformation at an initial value, and the ultrasonic wave vibration providing means provides the predetermined ultrasonic wave vibration, while the predetermined load is being applied by the load applying means, until the amount of wire transformation measured by the transformation amount measuring means reaches a predetermined set value.